Conventional worm gear sets employ a worm screw and a fixed tooth gear wheel. Although such worm gear sets are successful drive mechanisms at low speeds, their efficiency is limited due to the rubbing or sliding contact between the fixed teeth of the gear wheel and the screw thread of the worm screw.
By substituting rollers for the fixed teeth of the gear wheel of the conventional worm gear sets, the friction between the gear wheel (which because of such substitution would now be more appropriately referred to as a roller worm wheel) and the worm screw can be reduced, thereby improving the efficiency of the resulting worm drive system. Worm drive systems employing a worm screw and a roller worm wheel have been proposed in the past. Such systems can be divided into the following three categories.
The first category is characterized by roller worm wheels which employ radially arranged rollers (i.e., the axis of rotation of each roller lies in a plane which is normal to the axis of rotation of the worm wheel). The drive systems diagnosed in U.S. Pat. Nos. 626,515; 715,973; 747,463; 767,588 and 3,597,990 are exemplary of this first category. These worm drive systems have limited power transmitting capability and limited load carrying capacity because the radially arranged rollers make it difficult to employ the type of bearings (i.e., needle bearings) required to transmit high power and carry large loads. Because the rollers would have to extend into the worm wheel in order to be used in combination with needle bearings, the number of rollers which could be employed without causing interference between their associated bearings would be limited, thereby limiting power transmitting capability and load carrying capacity even if such bearings were used.
The second category is characterized by roller worm wheels which employ a set of radially arranged rollers and two or more sets of angularly arranged rollers (i.e., the axis of rotation of each roller forms an inclined angle relative to a plane which is normal to the axis of rotation of the worm wheel). The worm drive systems disclosed in U.S. Pat. No. 908,049; 1,060,933 and 3,820,413 exemplify this category. Due to their utilization of radially arranged rollers, these drive systems suffer from the same power transmitting and load carrying limitations as the first category discussed above. They also, however, suffer from a further limitation in that their worm wheels can only be used in combination with two worm screws of the same hand which, therefore, would rotate in opposite angular directions. Thus, the worm drive systems of the second category would have no utility in applications requiring two worm screws which rotate in the same angular direction.
The third category is characterized by a worm wheel which employs angularly arranged rollers only and a worm screw having a single double-cut screw thread. More particularly, the worm wheel is provided with two sets of rollers, the rollers of one set having full tips and the rollers of the other set having stepped tips. The worm screw is provided with a single screw thread having a first helical path generated to accept the rollers with the full tips and a second helical path generated to accept the rollers with the stepped tips. Because all of the rollers are only rolling in half threads, the power transmitting capacity and the load carrying capability of such a worm drive system is limited. Further, the worm drive system will only run in one direction to full capacity because when the direction of rotation of the worm screw is reversed the rollers with the stepped tips will be out of engagement with a thread surface and the rollers with full tips will engage a half thread surface only.